That's great! Does it give you a report on micro and macronutrients that you may be missing? We use my plate to do patient nutritional assessments.
We also have a food diary here. It's very easy to set up and use.
Oh, I forgot! There's a website, my plate.gov, that will track your daily food intake, give you tips on nutrition, diet, what you're getting too much of or not enough of. We used it in my Clinical Nutrition class.
1. The easiest way to keep from overeating is serve yourself smaller portions, then don't accept seconds. You need more protein and fiber to fill you up and keep you feeling fuller longer. When you fix your plate, half should be veggies, 1/4 should be protein (meat, chicken, fish, etc) and 1/4 should be grains, such as rice, whole grain pasta, etc.
2. Since you have a bad back, you should talk to your doctor, first, but I'd recommend trying yoga. It's, typically, suitable for all ages and levels of activity. There's even chair yoga. Walking is another exercise that nearly everyone can do. Aim for 30 minutes, at least 3 days/week; best for 30 minutes daily.
3. Those leftovers could actually serve you quite well for your lunch, as long as you're cooking the right things; just don't bring such huge portions. I've never worked at Walmart, but I'd guess that most of them have microwaves available for heating food. You can pop your plate into the microwave and in just a few minutes, have a good hot meal.
An alternative would be to bring yourself a large salad, with meat and/or boiled egg for the protein, and loaded with lots of veggies. A sandwich made with some type of meat, such as left over roast beef, chicken, turkey, etc (not processed luncheon meat) and loaded with lettuce, tomato, onion, sweet peppers on whole grain bread, would also work.
You could also make a protein shake to take with you and store it in the fridge till lunch time. A good one, is 1/2 cup fat free Greek yogurt, 1/2 avocado, 1 scoop any protein mix, 1/2 cup milk (low/no fat), 3-4 ice cubes. Blend well and pour into glass. I usually drink this when I make it, but it can keep in the fridge for a while, as well. If you don't like avocado, use whatever fruit you like.
Stop with the cookies and Little Debbie snacks. Those are some of the worst things you can eat!! Have an apple with 2 tbsp peanut butter for your snack or some grapes and piece of string cheese, some cherries, cut up veggies, a yogurt.... there are so many good choices. Some of them may take a bit of prep time, but it's worth it, in the long run.
The biggest thing is to watch your portion sizes, especially, eating at McDonald's, since they tend to "supersize" everything. If you must eat at McDonald's, remove at least 1/2 the bun from your sandwich and eat the meat and veggies with a fork. Order small french fries, instead of large, drink water instead of the soda.
Drink plenty of water throughout the day; water helps you stay full and when you're hydrated well, you don't tend to crave foods.
Try to use at least 10 min or so of your lunch break to get in a brisk walk, even if it's just walking around the parking lot.
Hope this can get you started and that you'll stick with us, so we can see how it goes for you.